This invention relates to signal communication systems, particularly telephone switching networks, in which conference bridges are employed to interconnect a plurality of communicating parties. More specifically, this invention relates to a system and method for improving the performance of such conference bridges.
Conference bridges are known which are employed in signal communication systems, particularly telephone switching systems, to permit more than two communicating parties to participate in a telephone conversation. In a typical conference bridge, the circuitry is designed in order to enable each participant in the conference to hear the sum of all of the participants' signals, minus that participant's contribution to the total. In such an arrangement, a summing network is typically provided for summing the electrical information signals from each of the individual participants' station sets, whether local or remote, and a plurality of subtraction circuits are provided each having the resultant sum signal as one input and the signal from one of the participants' station sets as the other input for supplying a partial sum signal to that participant.
In a typical telephone network, the circuits employed to interconnect the individual station sets (and trunk lines) are "two wire" circuits in which information flows bi-directionally on a pair of wires coupled to any given station set or trunk. The "two wire" signals are converted to "four wire" signals by means of circuits known as "hybrid" circuits at all junctions between the two wire circuit paths and the equipment, such as the station sets or the telephone switchboard. In operation, when information signals are to be transmitted between items of equipment, a first hybrid associated to the first equipment item carries the appropriate signal across the four wire to two wire interface and subtracts the expected effects of the transmitted signal from any signal present on the two wire interface. At the other junction of the two wire path a second hybrid circuit associated to the other equipment item (e.g., a station set) carries the two wire signal across the two wire to four wire interface in a similar manner. Proper operation of each hybrid circuit is predicated upon the assumption that each item of terminating equipment exhibits a given impedance value to the hybird. So long as the impedances of the individual terminating equipment items as viewed by the hybrid are well-known and well behaved, each hybrid circuit will function in a proper manner. However, if the impedance of a given item is not well-known or is not well behaved, the operation of both the associated hybrid and the entire system will be adversely affected. In the example given above, if the impedance of the two wire line at the junction between the first equipment item and that line is well known and well behaved, the expected effects of the transmitted signal are sufficiently predictable so that the result of the subtraction on the four wire side of the first hybrid comprises only the signal on the two wire side which was transmitted from the designated station set. However, if the impedance of the two wire line is not well-known or is not well behaved, the information signal on the four wire side of the hybrid, after the subtraction, will contain additional information corresponding to the signal transmitted from the first hybrid to the designated station set. This result has been conceptualized as a reflection of a portion of the signal sent to the designated station set off the hybrid circuit and back to the equipment on the four wire side.
This reflection phenomenon assumes critical importance when several ports of a conference bridge are in use in a telephone switching system, since the impedances of the two wire circuits seen by each conference port hybrid do not meet the assumption criteria noted above. Thus, when several ports of a conference bridge are in use, the combined reflections of the signals back to the conference bridge may lead to an unstable condition in which the conference bridge circuitry and the several ports form an oscillator, resulting in the generation of audible sounds which at best impair the intelligibility of the speech signals perceived at the several individual station sets and at worst may damage the hearing of the conference participants. In a time division multiplexed digital switching system, a conference bridge of the type noted above is even more prone to oscillation since there is a fixed delay in each signal path caused by the sampling and desampling process employed in such a system and since substantial phase shifting of the signals occurs in the filters associated to the sampling and desampling circuitry. Even if the conference bridge does not actually oscillate, operation close to the unstable conditions noted above impairs the quality of the perceived speech signals at the individual conferenced station sets, usually perceived as a "hollowness".
Qualitatively, the conference bridge and ports will oscillate at those frequencies for which the gain of the system comprising the conference bridge circuitry including the ports exceeds unity and for which the phase shift produced on these signals by the system cause the aggregate of the reflected signals to be in phase with the transmitted signal. If the gain at one potentially oscillatory frequency dominates, the system will oscillate at only that frequency with saturation generated harmonics thereof.